Corrosion resistant nickel-chromium-iron alloy

ABSTRACT

Directed to an alloy having improved oxidation resistance and particularly improved resistance to the effects of cyclic oxidation at high temperatures, e.g., 2,000* F., wherein the alloy contains about 21 to 25% chromium, about 58% to 63% nickel, about 1% to 1.7% aluminum and the balance essentially iron.

United States Patent Herbert L. Eiselstein;-

James C. I-Iosier, both of Huntington, W. Va.

[21] Appl. No. 5,691

[22] Filed Jan. 26, 1970 [45] Patented Sept. 21, 1971 [73] Assignee The International Nickel Company Inc.

New York, N.Y.

[72] Inventors [54] CORROSION RESISTANT NICKEL-CHROMIUM- IRON ALLOY 2 Claims, 2 Drawing Figs.

[52] U.S.Cl 75/171, 148/32 [51] Int. Cl C22c 19/00 [50] Field of Search 75/171, 170; 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 1,688,247 10/1928 Smith et a1 75/171 2,813,788 11/1957 Skinner 75/171 Primary Examiner-Richard 0. Dean Attorney-Maurice L. Pinel ABSTRACT: Directed to an alloy having improved oxidation resistance and particularly improved resistance to the effects of cyclic oxidation at high temperatures, e.g., 2,000 E, wherein the alloy contains about 21 to 25% chromium, about 58% to 63% nickel, about 1% to 1.7% aluminum and the balance essentially iron.

PATENTED m1 m 3607243 FlG.2

INVENTORS HERBERT L. EISELSTEIN JAMES C. HOSIER M1. PM

from?! CORROSION RESISTANT NlCKEL-CHROMlUM-IRON ALLOY At present there are many demands in industry for the provision of alloys resistant to the effects of oxidation including cyclic oxidation at very high temperatures, e.g., 2000 F. and higher. Such industrial applications require that the alloy employed have sufficient strength at the operating temperature to retain its fabricated shape and mechanical integrity over prolonged periods of time. Such applications include heat-treating baskets and fixtures, radiant furnace tubes, strand-annealing tubes and the like. An important commercial factor in connection with such industrial demands has to do with the cost of the alloy. Thus, alloys are available which are resistant to the effects of high temperatures, i.e., alloys which have earned the popular designation superalloys, but the cost of such alloys is so great that they are limited presently to exotic applications such as gas turbine blades and the like in which the high cost of the alloy can be justified in terms of the overall purpose involved. in many other applications the high cost associated with such alloys cannot be justified and as a result industry has been forced to use cheaper alloys and to replace the part involved frequently when it failed in use.

Another vexing factor which arises when service temperatures are increased from, for example, 1800 F. up to about 2000" F., is the discovery that materials which perform satisfactorily at 1800 F. failed unpredictably at 2000 F. Another fact developed by high temperature research is that materials which are satisfactory at the 1800 F. temperature level do not behave the same at 2000 F. The vagaries of performance in known materials becomes particularly acute when the alloys are subjected to cyclic periods of heating at the intended service temperature and cooling to room temperature. Such cyclic heating involves the formation of oxide scale on the alloy surface and in many alloys the scale becomes mechanically removed during the alternating cycles of heating and cooling, a condition which leads to further scaling and rapid failure of the alloy. in addition, it is found that in many alloys, particularly when exposed as thin sections, warping results during the cyclic oxidation test, a condition which leads again to accelerated failure in service of a part fabricated therefrom.

In order to overcome the complex and vexing problems encountered during exposure to high temperatures, particularly under cyclic conditions, while still providing a satisfactory alloy capable of production at moderate cost, a number of problems must be solved. Thus, the alloys sought must be capable of providing a tightly adherent oxide scale which is not removed by the stresses encountered during alternate heating and cooling. The alloy must be sufficiently strong, it must be workable both hot and cold so as to be provided in common mill forms including sheet and strip, and it must be weldable.

It is to the solution of the foregoing complex problems that the present invention is directed.

The objects of the invention will become apparent from the following description taken in conjunction with the drawing, in which: I

FIG. 1 is a reproduction of a photograph taken at approximately full size depicting the appearance of strip specimens made from the alloy of the invention and from two comparable commercial alloys at the end of a 1000 hour cyclic oxidation test, and

FIG. 2 is a reproduction of a photomicrograph taken at 100 diameters showing a transverse section of a strip specimen made from the alloy of the invention after being subjected to cyclic oxidation.

Generally speaking, the alloy provided in accordance with the present invention contains about 21% to about 25% chromium, about 58% to about 63% nickel, up to about 0./%

carbon, about 1% to about 1.7% aluminum, up to about 0.6%

titanium, up to about b 0.5%silicon, up to about 1% manganese, up to about 0.006% boron and the balance essentially iron. A preferred alloy provided in accordance with'the invention contains about 22.5% chromium, about 0.03% carbon, about 0.2% manganese, about 0.3% silicon, about 0.3% titanium, and about 1.3% to about 1.5% aluminum.

The contents of nickel, aluminum and chromium in the alloy are particularly important and each of these elements: must be maintained within the ranges specified to provide in the alloy its characteristic improved combinations of properties including improved resistance to oxidation at hightemperatures, e.g., 2000 F., together with the high strength over a range of temperature from room temperature to high temperature, high fatigue strength and hot and cold malleability. The alloy contains not more than about 0.1% carbon to avoid formation of carbides in the microstructure. Titanium may be employed in the alloy in amounts up to about 0.6% as a malleablizer. Desirably, the contents of manganese and silicon are low, i.e., not exceeding about 1% and about 0.5%, respectively, since higher amounts of these elementsare harmful to high temperature oxidation resistance and weldability, respectively. The alloy may contain up to about 0.006% boron, and melt additions of up to about 0.05% calcium and up to 0.1% magnesium may be made for deoxidation and malleabilization. The balance of the alloy is essentially iron including small amounts of impurities usually present in nickel-chromiumiron alloys, e.g., up to about 0.015% sulfur, up to about 0.03% phosphorus, up to about 0.5% copper, etc. The relatively high iron content in the alloy enables the use of standard ferroalloys inmelting the alloy thereby contributing to low cost. The alloy may satisfactorily be prepared by conventional airmelting techniques using standard melting furnaces including the induction furnace. Use of the more expensive vacuum melting procedure is unnecessary. The alloy is essentially nonagehardening and characterized by stable austenitic structure which develops no sigma phase during extended periods of heating, e.g., 300 hours, in a temperature range of 1 F. to 1400 F.

In order to give those skilled in the art a better understanding of the advantages of the invention the following examples are given.

EXAMPLE I A commercial scale melt was prepared in an induction furnace. The melt contained about 0.07% carbon, about 0.08% manganese, about 0.16% silicon, about 60.32% nickel, about 22,71% chromium about 1.28% aluminum, about 0.33% titanium, about 0.004% boron and the balance essentially iron. Various mill products including hot rolled rounds were prepared from the alloy by conventional procedures. In the form of %-inch-diameter hot-rolled rod annealed at 2100 F. for 1 hour and cooled, the alloy developed a yield strength (0.2% offset) of 34.6K.s.i. (kilo pounds per square inch), a tensile strength of 102 K.s.i. an elongation of 40% and a reduction in area of 61.7%. The Charpy V-notch impact value for the material in this condition was 136 foot-pounds.

Annealing tests conducted on the hot rolled material demonstrated that the yield strength, tensile strength and the grain size of the material were substantially unaffected as compared to the hot rolled properties until the annealing temperature exceeded 1800 F. Thus, the material annealed at 1800 F. had an average grain size of about 0.0008 inch, a yield strength of 62 K.s.i. a tensile strength of 114 K.s.i. and elongation of 42%. After a 2100 F. solution anneal, the grain size increased to 0.0035 inch average diameter.

Stress rupture tests conducted on the rod material after annealing at 2100 F. for 1 hour followed by air cooling indicated rupture lives at various temperatures as set forth in the following table 1. g

TABLE 1 'l'cnnllc '1 cmp Stress (K.s.i.) [or rupture in '1' I hours 1,000 hours 1,200" l' '17 27 1,100 1' 21 14.5 palm" 1- 14 9.8 1.500" I. I0 6.6 1,600" I. 7 4.6 l .1100" l- 3.4 2.3 2 you" r 1.6 1.05

Tensile properties determined on iii-inch diameter hotrolled rod annealed at 2,100 F. for 1 hour and air coiled were demonstrated at various temperatures as set forth in the following table 11.

TABLE II Yield Tensile Reduction Temperature, strength, strength, Elongation, in area, F. K s.i. 0.2% K s.i. percent percent Standard rotating beam fatigue tests were conducted upon bar stock of the aforementioned alloy at room temperature. Hot rolled stock three-fourths inch in diameter subjected to solution annealing at 2,100 F. for 1 hour followed by air cooling, and %-inch-diameter hot-rolled stock was annealed at 1,800 F. to provide test material. The results of the rotating beam fatigue tests are summarized in the following table 111.

The foregoing data demonstrated that the alloy has a high fatigue strength and high ratio of endurance limit to tensile strength as compared to nickel-chromium and nickel-chromium-iron alloys in the same price range.

it was found that '-inch plate made of the foregoing alloy can be welded to itself using a tungsten-arc argon-shielded welding process and using filler metal of matching compositions. Welds so made under conditions of severe restraint were sound as demonstrated by X-ray and side bend tests. Again, satisfactory autogeneous butt welds were made by the tungsten-arc argon-shielded process in /a-inch-thick sheet materials.

EXAMPLE 11 A further air-melted alloy containing about 0.06% carbon, about 0.09% manganese, about 0.17% silicon, about 60.25% nickel, about 23.35% chromium, about 1.26% aluminum, about 0.31% titanium, about 0.004% boron and the balance essentially iron, was prepared in a commercial scale induction furnace. A portion of the material was converted to strip having a thickness of about 0.05 inches. Test coupons from the strip having initial dimensions of about three quarter inch by 3 inch were subjected to cyclic oxidation testing in whichthe specimens were heated in a furnace for 15 minutes at 2000 F.

andthen withdrawnabove the furnace for 5 minutes for cooling in the ambient air. The test was conducted for 1000 hours total exposure period during which time specimens were removed periodically for weighing and visual examination. Comparison test coupons of a nickel-chromium-iron alloy containing about 76% nickel, about 0.04% carbon, about 15.8% chromium, and about 7.2% iron, and of an iron-nickelchromium alloy containing about 32% nickel, about 46% iron, and about 20.5% chromium were tested simultaneously. At the end of the l000-hour test period the coupons made from the aforementioned alloy within the invention were only slightly bent whereas the test coupon made of the nickelchromium-iron alloy disintegrated during the last 300 hours of exposure in this severe test and the test coupon made of the iron-nickel-chromium alloy became severely bent and twisted during the test. The appearance of the aforementioned samples at the end of the test is shown in the attached P10. 1. The upper left-handed picture depicts the disintegrated nickelchromium-iron alloy. The upper right hand picture depicts the severely bent and twisted iron-nickel-chromium alloy whereas the picture at the bottom of FIG.. 1 depicts the test coupon made from the alloy of the invention. The iron-nickel-chromium alloy exhibited a severe loss in weight during the course of the test; to wit, weight loss of about 300 milligrams per square centimeter of exposed surface. To the contrary, the alloy of the invention demonstrated substantially no weight change during the course of the test. Metallographic examination of the test coupon made of the alloy of the invention demonstrated that during the course of the severe test it developed a very strongly retentive oxide scale as depicted at diameters in FIG. 2 of the drawing. The ability of the alloy to develop a highly adherent oxide scale which seems to have a characteristic of being keyed to the metal surface by a uniform network of internally penetrating oxide apparently accounts for the marked resistance of the alloy to the destructive effects of cyclic oxidation. The thin section warping resistance of the composition is particularly marked as indicated by FIGS. 1 and 2 of the drawing.

Further tests on sheet material containing welds made of matching composition demonstrated that the welded specimens were also highly resistant to cyclic oxidation.

Tests in powdered graphite and in a commercial carburizing composition for extended periods at 1800 F. demonstrated that the alloy of the invention is highly resistant to carburization.

The special alloy provided in accordance with the invention is useful for the production of articles and parts required to be resistant to high temperature oxidation in service. Such articles and parts include heat-treating baskets and fixtures, radiant furnace tubes, strand annealing tubes, retorts, etc. Such articles and parts can be subjected in use to reducing conditions on one side and to oxidizing conditions of the other without encountering undue distortion and premature failure.

Although the present invention has been described in con junction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An alloy resistant to scaling when subjected to cyclic oxidation at high temperatures consisting essentially of about 21% to about 25% chromium, about 58% to about 63% nickel, up to about 0.1% carbon about 1% to about 1.7% aluminum, up to about 0.6% titanium, up to about 0.5% silicon, up to about 1% manganese, up to about 0.006% boron, and the balance essentially iron.

2. An alloy in accordance with claim 1 containing about 22.5% chromium, about 0.03% carbon, about 0.2% manganese, about 0.3% titanium and about 1.3% to about 1.5% aluminum.

Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Inventor(s) HERBERT L. E S

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer line 70 for "0./%" read --O.l%--.

line I, delete the "b" after "about".

Line 39, for "1400F." read --l400F.--.

Line 49, for "22,7l%" read -22.7l%.

Line 56, for "40%" read --49%--.

in Table I, first column, for "1,3000F." read l,300Fo o Line 15, for "colled" read --cooled-.

Signed and sealed this 4th day of April 1972.

ROBERT GOTTSCHALK Commissioner of Patents 

2. An alloy in accordance with claim 1 containing about 22.5% chromium, about 0.03% Carbon, about 0.2% manganese, about 0.3% titanium and about 1.3% to about 1.5% aluminum. 